FIG. 1 illustrates a typical optical communications system using coherent detection. In an optical communications system, a transmitter 2 generally comprises an encoder 4 for encoding a pair of data signals in accordance with a predetermined encoding scheme, and a modulator 6 for modulating a continuous wave (CW) carrier light in accordance with the encoded data signals. In the arrangement if FIG. 1, a pair of data signals (dx and dy) may be modulated onto respective orthogonal polarizations of the carrier light. The CW carrier light is typically generated by a laser 8 in a manner known in the art, and the modulator may be implemented using any of a variety of known modulator devices, such as phase modulators, variable optical attenuators Mach-Zehnder interferometers etc. The modulated optical signal appearing at the output of the modulator 6 is transmitted through an optical fiber link 10 to a coherent receiver 12.
A typical coherent receiver 12 typically includes a polarization beam splitter 14 for splitting the received optical signal into received X and Y polarizations, an optical hybrid 16 for mixing the X and Y polarizations with a local oscillator light, and a set of photodetectors 18 for detecting the optical power of each of the mixing products generated by the optical hybrid 16. An A/D converter block 20 samples each photodetector current, and the resulting sample streams are processed by a Digital Signal Processor (DSP) 22 to generate recovered signals Rx and Ry that correspond with the transmitted data signals dx and dy.
For achieving long distance optical signal transmission, at moderate spectral efficiencies, dual polarization Binary Phase Shift Keying (DP-BPSK) is commonly used to encode the data signals and modulate the carrier light in the transmitter. As is known in the art, BPSK encodes a single bit value (“0” or “1”) onto an optical carrier by modulating the carrier phase between two constellation points, that are separated by 180°. This is illustrated in FIG. 2A, which shows the BPSK constellation mapped onto the Real (Re)—Imaginary (Im) plane of a carrier light.
As is known in the art, other modulation schemes enable increased numbers of bits to be encoded into a symbol. For example, Quadrature Phase Shift Keying (QPSK) enables two bits to be encoded on each polarization of carrier light, by using a symmetrical 4-point constellation as may be seen in FIG. 3A. Other modulation schemes, such as Quadrature Amplitude Modulation (QAM) achieve even higher numbers of bits per symbol by modulating both the phase and amplitude of the carrier. However, as the number of encoded bits-per-symbol increases, the Euclidian distance between neighbouring constellation points decreases. For example, in the BPSK constellations shown in FIG. 2A, each constellation point is separated from its neighbour by a Euclidean distance corresponding to 180°. On the other hand, in the QPSK constellations shown in FIG. 3A, each constellation point is separated from its neighbour by a Euclidean distance corresponding to 90°.
In practice, Phase Shift Keying encoding and coherent detection typically suffers greater penalties due to phase distortion than due to amplitude distortion. This is illustrated in FIG. 2B for the case of BPSK. As may be seen in FIG. 2B, each transmitted symbol (X(0), X(1)) may be detected by the receiver as a multi-bit symbol estimate lying anywhere within a detection region 24. As long as the respective detection regions of each symbol are distinct, as in FIG. 2B, known Maximum Likelihood (ML) techniques based on minimum Euclidean Distance (ED) can accurately determine the transmitted symbol given the detected symbol estimate. However, if the phase noise becomes too large, the detection regions can overlap, as may be seen in FIG. 2C. In this case, if the detected symbol estimate lies in the overlapping portion of two detection regions, Maximum Likelihood (ML) techniques based on minimum Euclidean Distance (ED) cannot accurately determine the transmitted symbol. This increases the probability of error in the recovered symbols, and thus the recovered data streams Rx and Ry. As may be seen in FIG. 3B, while QPSK modulation increases the number of encoded bits per transmitted symbol relative to BPSK, it is less tolerant of phase noise at the receiver.
Techniques that increase noise tolerance remain highly desirable.